Parser fixes

[philippjfr]

Two small fixes for the parser, the first fixes passing dictionary literals by eliminating duplicate commas generated by the parser, fixing part of the issue described in #270, e.g. this is now allowed:

%%opts Scatter [fontsize={'title': '20pt'}]
hv.Scatter(np.random.rand(100,2), group='A')

Secondly it fixes a bug when a opts definition includes nested bracketing as described in #930, which described an issue where:

%%opts Curve (color=hv.Cycle(values=sns.color_palette('Set1')))
hv.Overlay([hv.Curve(np.arange(10)*i) for i in range(3)])

Was not evaluated correctly. This is now also correctly processed.

Note that this does not fix all issues mentioned in #270, e.g. aspect=dict(x=1, x=2, z=3) still fails to evaluate, but it provides a significant improvement in some cases.

[jlstevens]

Parser improvements, yay!

All I will ask is that you add a bunch of unit tests. There are already a bunch here but you cannot have too many parser tests.

[philippjfr]

Agreed, happy to add tests.

[philippjfr]

Added some tests now, which ended up helping me figure out further issues. Still not perfect but it actually let's you do a fair number of more complex expressions now.

[jlstevens]

Great to have more fixes and more tests! Merging.